Scientists believe the earth’s core is around 6,000 degrees celsius. where does this heat come from?

The heat in the core of the Earth has several sources including:
1- The heat produced from the decaying of radioactive elements
2- The heat that was originally produced during the formation and accretion of the planet itself (This heat has not yet faded)
3- The heat produced during friction when denser material and elements sink into the planet's center.